As described in Japanese Patent Application Laid-Open No. Hei. 10-318781 (hereinafter referred to as Patent Literature 1) or in Japanese Patent Application Laid-Open No. 2003-121206 (hereinafter referred to as Patent Literature 2), there is known an electromagnetic induction type encoder as in FIG. 1 showing an example of Patent Literature 2. The electromagnetic induction type encoder includes: a large number of scale coils 14 and 16 arranged on a scale 10 along a measuring direction thereof; and transmitter coils 24 and 26 and receiver coils 20 and 22 arranged on a grid (also referred to as a slider) 12 capable of moving relative to the scale 10 in the measuring direction. The electromagnetic induction type encoder detects an amount of relative movement between the scale 10 and the grid 12 from a flux change detected at the receiver coil via the scale coil when the transmitter coil is excited. In this figure, reference numeral 28 denotes a transmission controller, and 30 denotes a receiving controller.
As shown in FIG. 2, in order to reduce an offset, which is an excess signal, in such an electromagnetic induction type encoder, an offset has been reduced by placing the receiver coil 20 at a portion where magnetic fields generated by the transmitter coils 24 are cancelled out to be net zero (a central portion between the transmitter coils on the both sides thereof in the example of FIG. 2). Note that in Patent Literature 2, the second receiver coils 22 are also arranged on the both sides of the second transmitter coil 26 as shown in FIG. 3 in addition to the configuration formed by the first transmitter coils 24 and the first receiver coil 20 shown in FIG. 2.
However, this configuration requires three rows of scale coils, and the line length of the scale coils therefore becomes long. Thus, there is the problem that the generated induced current is attenuated due to an impedance of the scale coil itself and it is therefore difficult to obtain a strong signal.
In order to solve such a problem, the present applicant has suggested in Japanese Patent Application Laid-Open No. 2009-186200 (hereinafter referred to as Patent Literature 3) that a plurality of sets of transmitter coils 24A and 24B, receiver coils 20A and 20B, and scale coils 14A and 14B are arranged symmetrically with respect to the center of the scale 10, and one of the scale coils symmetrically positioned with respect to the scale center (14A, for example) has a positional relationship shifted from the other one of the scale coils (14B, for example) by a ½ phase of a scale pitch λ as shown in FIG. 4 of the present application corresponding to FIG. 6 in Patent Literature 3.
Furthermore, as shown in FIG. 5, it is conceivable to enable absolute position measurement by placing two sets of a track including scale coils, a transmitter coil, and a receiver coil in a scale width direction (grid width direction) with different scale pitches of λ1 and λ2. The two sets includes a set formed by a transmitter coil 24-1 on the lower side of the figure, and scale coils 14-1a and a receiver coil 20-1 at the scale pitch λ1 on the upper side of the figure and a set formed by a transmitter coil 24-2 on the upper side of the figure, and scale coils 14-2a and a receiver coil 20-2 at the scale pitch λ2 on the lower side of the figure. In this figure, reference numeral 14-3 denotes a coil for connecting the scale coil 14-1a with the scale coil 14-2a (referred to as a connecting coil).
With the configuration of FIG. 5, however, the receiver coil 20-1 (20-2) and the transmitter coil 24-1 (24-2) need to be placed at positions spaced apart from each other in order to reduce a direct crosstalk amount from the transmitter coil 24-1 or 24-2 to the receiver coil 20-2 or 20-1 on the grid 12. Thus, a length of the scale coils on the scale 10 (a length of the scale coil 14-1a+a length of the scale coil 14-2a+a length of the connecting coil 14-3) becomes long, resulting in attenuation of generated induced current Ia due to the impedance of the scale coil itself. Thus, there is the problem that a strong signal is difficult to be obtained.
Moreover, if it is attempted with the configuration of FIG. 5 to reduce the encoder width, for example, by reducing a space between the scale coils 14-1a and 14-2a, a magnetic field generated by the driving of the transmitter coil also directly affects the scale coil directly facing the receiver coil. As a result, there is generated an induced current component in a direction opposite to that of the normal induced current flowing through the scale coil, thereby reducing an induced current generated in the scale coil. This leads to the problem of a decrease in signals detected at the receiver coil.
A description thereof will be given below.
FIG. 6 shows an operation of detecting the scale coils 14-1a at the scale pitch λ1 on the upper side of FIG. 5 by the receiver coil 20-1. As shown in the figure, the principle of the detection is essentially such that a magnetic field generated by the driving of the transmitter coil 24-1 with a driving current ID leads to the generation of the induced current Ia at the scale coil 14-2a and a magnetic field generated by the induced current Ia flowing through the scale coil 14-1a via the connecting coil 14-3 is then detected by the receiver coil 20-1. As the scale coil 14-1a approaches to the transmitter coil 24-1, however, an induced current component Id in a direction opposite to that of the induced current Ia is generated at the scale coil 14-1a due to the magnetic field generated by the driving of the transmitter coil 24-1. As a result, the total induced current at the scale coil 14-1a becomes (Ia−Id), i.e., a reduction by an amount of Id. In other words, the induced current (Ia−Id) corresponding to a difference between the induced current component Ia via the scale coil 14-2a and the induced current component Id (in the direction opposite to that of Ia) generated by the magnetic field directly entered into the scale coil 14-1a from the transmitter coil 24-1 are generated at the scale coil 14-1a. 
On the other hand, FIG. 7 shows an operation of detecting the scale coil 14-2a on the lower side of FIG. 5 at the scale pitch λ2 by the receiver coil 20-2. As shown in this figure, the principle of the detection is essentially such that a magnetic field generated by the driving of the transmitter coil 24-2 with the driving current ID leads to the generation of the induced current Ia at the scale coil 14-1a and a magnetic field generated by the induced current Ia flowing through the scale coil 14-2a via the connecting coil 14-3 is then detected by the receiver coil 20-2. As the scale coil 14-2a approaches to the transmitter coil 24-2, however, the induced current component Id in the direction opposite to that of the induced current Ia is generated at the scale coil 14-2a due to the magnetic field generated by the driving of the transmitter coil 24-2. As a result, the total induced current at the scale coil 14-2a becomes (Ia−Id), i.e., a reduction by an amount of Id. In other words, the induced current (Ia−Id) corresponding to a difference between the induced current component Ia via the scale coil 14-1a and the induced current component Id (in the direction opposite to that of Ia) generated by the magnetic field directly entered into the scale coil 14-2a from the transmitter coil 24-2 are generated at the scale coil 14-2a. 
Further, if it is attempted with the configuration of FIG. 5 to reduce the encoder width, for example, by reducing the space between the scale coils 14-1a and 14-2a, the following disadvantage also occurs.
In other words, as shown in FIG. 8, a magnetic field generated by the induced current (Ia−Id) flowing through the scale coil 14-2a directly affects the receiver coil 20-1, and a crosstalk current component Ic (Ic1 at the left end side of the scale on the left side of FIG. 8) therefore flows through the receiver coil 20-1.
In a case of the scale having the two-track configuration with different scale pitches as shown in FIG. 5, the crosstalk current component generated at the receiver coil 20-1 varies depending on a position of the scale (Ic2 at the right end side of the scale on the right side of FIG. 8). Thus, the crosstalk current component, which varies depending on a position in the measuring direction of the scale, is superimposed on a true position detection signal, thereby causing a problem of affecting a measurement accuracy (in particular, a wide range accuracy) over the entire length of the scale.
In the scale of FIG. 5 having tracks with the scale pitch λ1 and the scale pitch λ2 (λ1<λ2 in this example) respectively, if the scale at the scale pitch λ1 is detected (measured) by the receiver coil 20-1 as in the example of FIG. 8, the wide range accuracy thereof has errors in a plus direction as shown in FIG. 9 due to the effect of the scale at the scale pitch λ2. If the scale at the scale pitch λ2 is detected (measured) by the receiver coil 20-2, on the other hand, the wide range accuracy thereof has errors in a minus direction as shown in FIG. 9 due to a cause similar to that described above, i.e., the effect of the scale at the scale pitch λ1.